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Ok, only 30m range - is that enough? No? Then modify the transmitter with an additional antenna and mount it on a wall facing the garden at a highish position.

A couple of heaters in the 'den' - one low power thermostatic (800W max) for background heat, and a fan heater for a quick heat-up. Add to each remote socket a signal lamp of some sort - a differently-coloured LED in a small table lamp will suffice. That way you have an indication from the house as to what's on - red glow for 'hot' and green for background...